Generally, an automatic transmission for a vehicle is configured to detect a driving state of the vehicle from a detecting means disposed at an appropriate location of the vehicle and automatically control an operation of a gear train consisting of a combination of planetary gear sets in a transmission control unit (TCU) on the basis of the driving state to automatically perform gear-shifting.
Therefore, it is preferable that the gear train used in the automatic transmission is configured to implement more shifting stages in order to improve power performance and a fuel consumption rate.
In addition, since durability, power transfer efficiency, size and weight, among other traits, of the gear train are significantly changed depending on a combination method of the planetary gear sets even though the same number of shifting stages are implemented, an effort to develop a gear train that is firmer, minimizes power loss and is smaller has been continuously conducted.
Further, in designing the gear train of the automatic transmission as described above, a one-way clutch acting as a planetary gear reaction force element in a 1-stage and a low and reverse (LR) brake acting as a planetary gear reaction force element in the 1-stage and an R-stage may be provided.
However, since an automatic transmission according to the related art may include both of the brake and the one-way clutch, functions of the one-way clutch and the LR brake overlap with each other in the 1-stage, and drag loss is generated from a 2-stage to a high stage.